1. Technical Field
Methods for forming thin films of semiconductor devices are disclosed, and more specifically, methods for forming thin films of semiconductor devices are disclosed which solve the problems associated with growth rate variations during the process of forming the films. An apparatus is disclosed for the film forming process which comprises a metal dome and a vertically movable susceptor having a resistor type heater disposed in a chamber.
2. Description of the Related Art
Generally, thin films formed on semiconductor substrates are formed by performing deposition process in a chamber.
FIG. 1 is a cross-sectional diagram illustrating a chemical vapor deposition apparatus in which processes are performed one wafer at a time.
Referring to FIG. 1, a reactor comprises a metal dome 11 and a chamber 13. A heater 17 for heating a semiconductor substrate 27 is mounted in a susceptor 15 composed of graphite.
The susceptor 15 moves vertically. When the semiconductor substrate 27 is inserted in the chamber, the susceptor 15 is lowered to a predetermined position, and the substrate 27 is moved through a chamber entryway 19 and placed on the susceptor 15 by a robot arm. Thereafter, the susceptor 15 is lifted to a predetermined position, and source gas is injected through a gas outlet 21 into the chamber whereby, chemical vapor deposition reaction is started to grow a thin film on the semiconductor substrate.
After the growth process of the thin film is completed, the susceptor 15 is lowered again, and the substrate 27 having a thin film thereon is moved through the chamber entryway 21 out of the chamber by the robot arm and then a new semiconductor substrate is placed on the susceptor 15, and the first cycle of process is completed.
However, the thin films grown on the substrate do not have the same thickness although the same process is performed to grow thin film on different semiconductor substrates.
FIG. 2 is a graph illustrating the thickness of polysilicon film formed on wafers processed according to insertion sequence using the apparatus shown FIG. 1. FIG. 2 shows that a desired thickness is obtained from the 9th wafer.
Eight wafers previous to the 9th wafer are used as dummy substrates, which is very inefficient.
Although the growth process is performed under the same conditions, each thin film has different thickness due to difference between the actual process environment and the process conditions set by apparatus operator.
The growth speed of polysilicon is sensitive to temperature. The actual surface temperature of the substrate differs from the set temperature because the susceptor 15 moves vertically as shown in FIG. 1 and the distance between the dome 11 and the substrate 27 is not constant.
The dome 11 which is composed of metal has considerable reflectivity in a wavelength range including wavelength of infrared rays and the semiconductor substrate 27 also has considerable, respectively.
The surface temperature of the substrate 27 is affected by heat reflected from the dome 11 as well as heat conducted from the resistor type heater 17.
The heat reflected from the dome 11 becomes weaker as the distance between the dome 11 and the substrate 27 becomes greater.
Here, the susceptor stays stopped in idle state before the thin film growth process starts. However, when the process starts, the susceptor 15 begins to move up and down repeatedly and the surface temperature of the substrate 11 drops.
The temperature drop of substrate is sensed by temperature sensors such as thermocouple (not shown), and the heater 17 is further heated to maintain the desired temperature of substrate. However, the temperature stabilization requires much time because the resistor type heater 17 does not immediately react to increase in power applied to the heater 17.
As a result, when the thin film growth process starts, each of initially inserted wafers has thin films having different thickness formed thereon because the process is not performed at the desired set temperature.
As described above, because thin films formed on semiconductor substrates initially inserted one at a time in the apparatus does not have desired thickness, the conventional method for forming thin films of semiconductors reduces the productivity of semiconductor devices, thereby degrading yield thereof.
Accordingly, a method is disclosed for forming thin films of semiconductor devices, comprising performing a preheating process before the main process to stabilize the surface temperature of susceptor so that the temperature drop due to vertical movement of the susceptor is compensated.
A method for forming a thin film on a semiconductor substrate is disclosed, wherein the semiconductor substrate is subjected to a thin film formation process in a thin film formation apparatus containing a chamber, a susceptor vertically movable in the chamber and a heater disposed within the susceptor, the method comprising a preheating process for stabilizing the internal temperature of the chamber by vertically moving the susceptor a predetermined number of times prior to the thin film formation process.
It is preferable that a wall and ceiling of the chamber comprises metal, and the preheating process is performed under inert gas atmosphere with a dummy substrate mounted on the susceptor. The preheating process is performed using a source gas selected from the group consisting of SiH4, Si2H6, Si2H2CI2, GeH4, Ge2H6, Ge2H2CI2 and combinations thereof.
It is preferable that the number of vertical movements of the susceptor is determined by the factors comprising a stop position, a time period at the stop position of the susceptor and combinations thereof so that internal temperature of the chamber is stabilized. The susceptor moves vertically 1 to 30 times during the preheating process. The temperature of semiconductor substrate is stabilized at a temperature ranging from 600 to 800xc2x0 C. after the preheating process.
A method for forming a thin film on a semiconductor substrate is disclosed, wherein the semiconductor substrate is subjected to a thin film formation process in a thin film formation apparatus containing a chamber, a susceptor vertically movable in the chamber and a heater disposed within the susceptor, the method comprising performing a preheating process, prior to a thin film formation process, for stabilizing the internal temperature of the chamber by setting the temperature of the heater higher than the temperature required for the thin film formation process for a predetermined time period sufficient to compensate for a temperature drop due to vertical movement of the susceptor in the thin film formation process.
It is preferable that the temperature of the heater during the preheating, process is set 100 to 200xc2x0 C. higher than a temperature required during the thin film formation process.
The preheating process is performed under inert gas atmosphere with a dummy substrate mounted on the susceptor. The preheating process is performed using a source gas selected from the group consisting of SiH4, Si2H6, Si2H2CI2, GeH4, Ge2H6, Ge2H2CI2 and combinations thereof.
Generally, the heater is used at a temperature ranging from 700 to 1000xc2x0 C.
A method for forming a thin film on a semiconductor substrate is disclosed, wherein the semiconductor substrate is subjected to a thin film formation process in a CVD chamber, the method further comprising a preheating process for stabilizing the internal temperature of the chamber by vertically moving the susceptor a predetermined number of times prior to a thin film formation process.
A preheating process comprising vertical movement of susceptor is performed before the thin film formation process to have the surface temperature of susceptor to approach the surface temperature required during the actual process. This prevents the thin films formed on the semiconductor substrate from having difference in thickness and property when the formation process is performed at unstable temperatures in a CVD apparatus having a vertically moving susceptor, a resistor type heater and a metal dome.